The invention concerns an X-ray analysis apparatus, a curved Bragg reflector for use in the apparatus, a method for manufacturing the curved Bragg reflector and an apparatus for carrying out the method, the apparatus having
a source emitting X-rays PA1 a sample to be analyzed PA1 a detector sensitive to X-ray radiation PA1 beam-shaping and/or beam-limiting means PA1 a curved multilayer Bragg reflector disposed in the optical path between the source and the sample including a periodically repeated sequence of layers, PA1 A is the individual layer having the higher density and PA1 B the individual layer having the lower density.
wherein a period comprises at least two individual layers A, B having differing index of refraction decrements .delta..sub.A.noteq..delta..sub.B and with thicknesses d.sub.A and d.sub.B, PA2 wherein the period thickness, e.g. the sum d=d.sub.A +d.sub.B +d.sub.C + . . . of the individual layers A, B, C, . . . of one period, changes constantly along an x direction and PA2 wherein the reflector is curved in such a fashion that it forms a partial surface of a paraboloid at the focal line or focal point of which the source or an image of the source is disposed so that a parallel beam is produced subsequent to reflection.
An X-ray analysis apparatus of this kind is known in the art from WO 95/22758.
X-ray analysis apparatus include, e.g. X-ray spectrometers and X-ray-diffractometers and serve for the non-distructive analysis of solid, powder and liquid samples. Diffractometers, in particular powder diffractometers, normally utilize focussing beam configurations to guarantee efficient use of the X-ray beam illuminating the sample. These kinds of apparatus use, among other things, multilayer reflectors at which Bragg reflexion of the incident light occurs in order to monochromatize the X-ray radiation from the source. However, if reflectors consisting of planar homogeneous multilayers are used, the Bragg condition only holds for one single incident angle per irradiated wavelength so that the incident beam must be highly parallel.
In contrast thereto, the so-called graded multilayer mirror provides an improvement thereto with which the layers utilized exhibit a monotonically increasing period thickness on a flat substrate to make a divergent beam of incident radiation monochromatic.
An additional substantial improvement is achieved through use of a curved multilayer mirror as described in the above quoted WO 95/22758.
The curvature of this parabolic mirror is thereby tuned to a particular wavelength so that, in addition to a monochromatization, the incident radiation is focussed and a larger solid angle can be accepted from the source with the exiting beam made parallel.
This type of curved graded multilayer mirror is made by successive deposition of layers of laterally varying thicknesses on a flat substrate, generally a silicon wafer, with subsequent bending of the substrate having the multilayer reflector introduced thereon and glueing of this configuration onto a usually curved substrate holder, usually comprising aluminium or made from invar.
A curved reflector of this kind has, however, the disadvantage of being highly sensitive to, among other things, extremely slight geometric errors, since the radiation from the source impinges on the surface of the mirror at grazing incidence angles in the order of 1.degree.. For this reason, even extremely small dust particles or irregularities on the surface of the substrate have devastating effects on the required shape of the mirror. These effects are discussed e.g. in J. Phys. D. Appl. Phys. 28 (1995) A 270 through A 275.
Additional substantial errors due to stresses or relaxation effects in the substrate are present in the edge regions. Even a deviation in the order of 30" from the intended parabolic curve which would be caused by a shape deviation of 10 .mu.m over a length of approximately 60 mm, leads to substantial angular errors which affect the divergence and homogeneity of the reflected beam and its photon flux density.
Accordingly, even a particle having a size of a grain of dust disposed on the surface of the substrate holder is sufficient to significantly distort the multilayer mirror introduced thereon and thereby the overall optics. Stresses and strains present on the ends of the wafer associated with an inhomogeneous force distribution between the edge regions and the center, cause the edges to extend to a greater or lesser degree substantially in a straight fashion away from the center or even curve in the wrong direction so that, in these regions, incident light is reflected in a wrong direction.
An additional, non-negligible source of error is caused by the use of glue for attaching the substrate with the associated multilayer onto a mechanical holder. The loading with X-ray radiation during operation often leads to a foaming of the glue and thereby to a deformation of the entire mirror surface so that the associated reflector can no longer be used.
A high degree of manufacturing effort and skill is required in order to produce a curved multilayer Bragg reflector of the conventional kind according to WO 95/22758. The initially flat "mirror face surface" is introduced onto a curved reference surface made from mirror glass, using an optical attachment procedure, and remains fixed in this location with the correct curvature by means of adhesion. Glueing onto a suitable substrate holder is then carried out.
Finally, the conventional curved multilayer Bragg reflector only brings an increase in reflected photon flux density of a factor of 6 by rendering the incident diverging beam parallel using the same arrangement as described in M. Schuster and H. Gobel, J. Phys. D: Applied Physics 28 (1995) A270-A275. This is disadvantageous, since the theoretically achievable value is an improvement of a factor of 30. The difference between the actual and the theoretical effect can be explained by the above mentioned geometric errors due to the manufacturing and construction of the X-ray mirror.
In contrast thereto, it is a purpose of the present invention to create an X-ray analysis apparatus of the above mentioned kind with as little technical effort and expense as possible with which the transmission is substantially improved and the reliability and lifetime of the components is substantially increased.